Most boat batteries are 12-volt and can be charged using another boat’s battery or a battery charger. Marine batteries are usually dual purpose–they provide a burst of power for starting engines as well as a constant source of power for running boat electronics.
Locate the battery and expose the leads. The battery may be behind a hatch and inside a water-resistant case. Open the hatch and case. If the battery is in the bilge with a fuel tank or oil tank, make sure you allow any fumes to fully dissipate before you try to work with the battery.
Turn off the ignition and power main switch. If you have a power main switch, make sure it is switched to the off position.
Identify the positive and ground terminals. The positive terminal will be marked with “+” and is sometimes colored red or orange. The ground or negative terminal should be marked with “-” and is usually colored black.
Attach the positive lead. Always connect the leads when the charger source is unpowered. Start by connecting the positive lead to the positive post of the battery to be charged. Next attach the same cable to the positive lead on the charger.
Higher-end chargers have several charging options.
Attach the ground lead. Attach the ground lead to the ground post on the charger. Attach the same cable to the ground lead on the battery to be charged. Attaching the lead cables in this order minimizes the risk of fire or explosion.
Turn on the charger. Electronic chargers have several options for charging a battery. Check with the manufacturer of your battery to determine what type of charge to use. Trickle charge–low voltage and low amperage–is the safest choice if you do not know what type of charge to use.
Charge the dead battery for about three minutes.
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Turn off the charger and remove the lead cables. Remove the cables in the reverse order that you put them on.
Start the engine. Don’t forget to turn your power main back on.